The present invention relates to a semiconductor substrate having a semiconductor thin film formed at one surface side of a substrate body a separation layer, a thin film semiconductor device formed by using the semiconductor substrate, a manufacturing method of the semiconductor substrate and the thin film semiconductor device, and an anodizing apparatus.
In the technical field of thin film semiconductor devices such as solar batteries, for example, there has been promoted such a research that a semiconductor thin film of silicon is formed on the substrate body of a substrate of silicon (Si) through a porous separation layer, and then the semiconductor thin film is separated from the substrate body through the separation layer to reuse the semiconductor substrate (Japanese Unexamined Patent Publication No. Hei-8-213645). The method can contribute to resource-saving and cost-down. In order to reuse the substrate body as described above, it is required to easily separate the semiconductor thin film and the substrate body from each other, and it has been considered that the separation layer is formed of a porous layer whose porosity is varied.
For example, the thin film semiconductor device can be formed as illustrated in FIGS. 1A to 1E. That is, a substrate body 11 made of p-type single crystal silicon having resistance of 0.01 to 0.02 .OMEGA.cm is prepared (see FIG. 1A); and a porous separation layer 12 is formed at one surface side of the substrate body by anodization (see FIG. 1B). In the anodization process, current is supplied to the substrate body 11 serving as an anode with electrolyte. As shown in FIG. 2, for instance, the current is supplied while an electrolyte tank 41 is disposed at the one surface side of the substrate body 11 and an electrode 42 is disposed in the electrolyte tank 41. Alternatively, as shown in FIG. 3, the current supply is performed while the substrate body 11 is disposed between two electrolyte tanks 43 and 44 and electrodes 45 and 46 are disposed in the electrolyte tanks 43 and 44, respectively.
In the anodization process, a low-porous layer of low porosity is formed on the surface by supplying current at a low current density of 0.5 to 3 mA/cm.sup.2 for 8 min. Thereafter, a middle-porous layer of intermediate-level porosity is formed inside by supplying current at a middle current density of 3 to 20 mA/cm.sup.2 for 8 min., and then a high-porous layer of high porosity is formed inside the middle-porous layer by supplying current at a high current density of 40 to 300 mA/cm.sup.2 for several seconds. After forming a separation layer 12, a heat treatment is performed to form a semiconductor thin film 13 on the surface of the separation layer 12 (see FIG. 1C). Next, an adhesive substrate 15a is adhesively attached to the surface of the semiconductor thin film 13 through an adhesive layer 14a, and then pulled to separate the semiconductor thin film 13 from the substrate body 11 and transfer it to the adhesive substrate 15a (see FIG. 1D). The separation layer 12 adhering to the separated semiconductor thin film 13 is removed therefrom, and an adhesive substrate 15b is adhesively attached to the semiconductor thin film 13 through an adhesive layer 14b, thereby achieving a thin film semiconductor device such as a solar battery or the like (see FIG. 1E). On the other hand, the substrate body 11 is used again to form a semiconductor thin film 13 after the separation layer 12 is removed therefrom.
However, since single crystal silicon constituting the substrate body 11 has a cleavage face, the mechanical strength of the substrate body 11 is low, and the substrate body 11 is easily broken at the cleavage face even by small external force. Upon repetitive use of the substrate body 11, the mechanical strength of the substrate body 11 is further lowered due to increase of crystal defects through a heat treatment. Furthermore, if the substrate body 11 is handled with no stress in order to increase the recycle frequency or if a temperature increasing time and a temperature decreasing time are set to longer values so that no crystal defect occurs, a long time is needed for the manufacturing process. Accordingly, there has been such a problem that it is difficult to increase the recycle frequency of the substrate body 11.
In order to reuse the substrate body 11, the separation layer 12 remaining on the surface of the substrate body 11 must be removed after the semiconductor thin film 13 is separated, thereby keeping a good surface condition. Therefore, an etching treatment, and if occasion demands, a surface polishing or electrolytic polishing treatment are needed. Therefore, the repetitive recycle causes reduction of the thickness of the substrate body 11, and thus the recycle frequency is limited. In addition, the number of steps such as the surface polishing step, etc. to promote the recycle is increased, so that the manufacturing cost is increased.
In addition, if an elongated substrate 11 is achieved to obtain a large area, a single crystal silicon ingot having a cylindrical shape must be cut out along its longitudinal direction, so that a larger unusable portion occupies in the ingot, that is, there occurs a problem that material is wasted.
As means of solving these problems, it may be considered that the substrate 11 is composed of sapphire. Sapphire has high strength, high rigidity, high wear resistance, high heat resistance, high abrasion resistance, and high chemicals resistance, and it is well known as a material constituting a reusable semiconductor monitor wafer. Also, it can provide large-aperture single crystal, and can provide a large-area thin film semiconductor device. Further, it was reported by Manasevit et al. in 1964 that a single crystal silicon layer can be formed on the surface of sapphire.
However, in order to separate the large-area semiconductor thin film 13 from the substrate body 11, the separation layer 12 having uniform porosity over the large area must be formed on the surface of the substrate body 11. The anodization process is suitably used as a method of forming such a separation layer 12, and it is preferable that a silicon layer is formed on the surface of the substrate body 11 and made porous by the anodization process. However, sapphire is an insulator unlike p-type silicon which has been hitherto used as a constituent material of the substrate body 11, so that no current can be passed therethrough by a conventional anodizing apparatus shown in FIG. 2 or 3 and thus it cannot be made porous. That is, such a manufacturing problem occurs when the substrate body 11 of sapphire is used to solve the above problem.